Acoustic Plate Mode Research Articles

Enhanced acoustofluidic mixing in a semicircular microchannel using plate mode coupling in a surface acoustic wave device

In this study, we present enhanced acoustic mixing inside a semicircular microfluidic channel via acoustic streaming induced by acoustic plate modes (APMs) of a 500-μm-thick double-side polished lithium-niobate (LiNbO3) substrate. We demonstrated that the APMs supported by the LiNbO3 substrate can be excited by an interdigitated transducer (IDT) at higher frequencies, in addition to conventional surface acoustic waves (SAWs), and can generate stronger acoustic streaming flow (ASF) and stirring effects. Consequently, rapid and enhanced mixing processes were observed in the channel. We conducted full-wave modeling of the device with finite element (FE) simulation to elucidate the acoustic process and mixing behaviors of the APM-based approach. Subsequently, we fabricated the acoustofluidic device using a standard photolithographic process and a 3D-printed replica molding method. The measured excitation spectrum of SAW and APM agrees with the numerical prediction; additionally, experiments on mixing and 2 µm particle motion in the continuous channel flow validate the ability of the device to enhance micromixing using APM frequencies compared with that using SAWs in our device. Our approach enables distinctive APMs to be fully coupled to the SAW-based acoustofluidic platform, which potentially enables an integrated design for the use of SAW and APM frequencies, acoustic energy, and wave fields in a single system. Our results also suggest innovative applications of the conventional SAW acoustofluidic devices to extended frequency regimes via APMs without reducing the substrate thickness and IDT pitch.

Acoustic Plate Mode sensing in liquids based on free and electrically shorted plate surfaces

The sensing behavior to liquids for Acoustic Plate Modes (APMs) propagating along 64°Y, 90°X LiNbO3 plate was investigated vs. two electric boundary conditions. The changes in the APMs phase velocity and attenuation were measured upon exposure to different liquids wetting one of the surfaces of the plate, either free or electrically shorted by a thin conductive Al layer. The experimental data confirm that the presence of a metallic layer covering one of the plate surfaces affects the viscosity and temperature sensitivity of the device. The differences between the sensor response for various liquids, with free or metalized faces, are interpreted in terms of the APM polarization.

Temperature characteristics of acoustic modes in SiO2, LiNbO3, LiTaO3, Bi12GeO20, and Bi12Si20 piezoelectric crystal plates

Temperature coefficients of delay of zero- and higher-order acoustic plate modes in the most widespread piezoelectric crystals are calculated and measured. It is shown that, along with the well-known dependence of these coefficients on material constants (typical of surface and bulk acoustic waves), their values for plate modes depend also on the mode number, plate thickness, acoustic wavelength, and thermal expansion coefficient of the crystal across the plate thickness, which allows one to change the temperature sensitivity of plate modes in wide limits without changing the material and orientation of the crystal. For some combinations of the aforementioned parameters, very small temperature changes Δt ~ 10–3°C are detected.

Aspects of using acoustic plate modes of higher orders for acoustoelectronic sensors

The results from experimental studies of the sensor properties of acoustic modes in LiNbO3 plates with thicknesses h/λ from 0.7 to 5 (where h is thickness and λ is wavelength) are discussed. It is shown that the mode responses are not always equal when liquid is applied to the upper and lower surfaces of a plate. When h/λ ≤ 1, the higher harmonics of lower order modes n are localized near the surface by transducers and interfere with the fundamental harmonics of higher order modes, violating the response symmetry. When h/λ ≥ 1, no such interference is observed, since the higher order modes are localized in the central part of the plate while the lower order modes are localized near the surface with transducer, again violating the response symmetry. The development of sensors for gases and liquids based on higher order modes thus becomes possible only with thin plates at frequencies that exclude interference.

General properties of the acoustic plate modes at different temperatures

Using acoustic plate modes with SH-polarization and quartz crystal with Euler angles 0°, 132.75°, 90°, as an example, general properties of the acoustic plate modes at different temperatures are studied theoretically and experimentally in the range from −40 to +80°C. It is shown that in addition to well-known parameters responsible for temperature characteristics of acoustic waves the temperature coefficients of the acoustic plate modes depend on the mode order n, plate thickness h/λ, and expansion of the plate in direction of its thickness (h – thickness, λ – acoustic wavelength). These properties permit the mode sensitivity to be increased or decreased without replacing plate material and orientation.

Quasi-Linear Polarized Modes in Y-Rotated Piezoelectric GaPO4 Plates

The propagation of both surface and flexural acoustic plate modes along y-rotated x-propagation GaPO4 piezoelectric substrates was studied for several y-cut angles: the phase velocity and coupling coefficient dispersion curves were theoretically calculated for two different electroacoustic coupling configurations. The investigation of the acoustic field profile across the plate thickness revealed the presence of thin plate modes having polarization predominantly oriented along the propagation direction, and hence suitable for operation in liquid environment. These modes include the linearly polarized Anisimkin Jr. and the quasi longitudinal plate modes, AMs and QLs, showing a phase velocity close to that of the longitudinal bulk acoustic wave propagating in the same direction. The temperature coefficient of delay (TCD) of these longitudinal modes was investigated in the −20 to 420 °C temperature range, in order to identify thermally stable or low TCD cuts. The power flow angle, i.e., the angle between the phase and group velocity vectors, was also estimated to evaluate the substrate anisotropy effect on the acoustic wave propagation. The GaPO4 intrinsic properties, such as its resistance to high temperature and its chemical inertness, make it especially attractive for the development of acoustic waves-based sensors for applications in harsh liquid environment.

Sensing properties of the anisimkin Jr. acoustic modes in uncoated ST-quartz plates

The Anisimkin Jr. (AN) acoustic plate modes having quasi-longitudinal (QL), quasi-shear-vertical (QSV) and quasi-shear-horizontal (QSH) polarizations are studied experimentally in uncoated ST,X-quartz plates with normalized plate thicknesses h/λ = 1.485 and 1.67 (where h is thickness and λ is wavelength). The sensitivities of the modes toward mass loading (water vapor) are measured at different temperatures in the range 10.6°C to 23°C and compared with that of the Rayleigh SAW. The sensitivity of the same modes toward temperature is measured from 15°C to 85°C and compared with those of the bulk waves propagating in the same direction. Acoustic plate modes with dominant responses toward humidity or temperature are found.

Characteristic features of excitation and propagation of acoustic modes in piezoelectric plates

Excitation and propagation of the I.V. Anisimkin and Lamb acoustic plate modes of various orders n has been experimentally investigated using, as an example, rotated 128° YX LiNbO3 piezoelectric plates. It is shown that (1) an increase in the number of fingers of the interdigital transducers improves the frequency resolution of the neighboring modes with close velocities, (2) the use of aluminum as a finger material reduces the ripple of the mode amplitude-frequency characteristics (AFC)n as compared to that in the case of gold fingers, and (3) the propagation loss increases with the distance and mode number n. It has been found that group velocities (vg)n and phase velocities (vp)n of the modes excited and received on the same surface of the plate do not change with distance starting, at least, from a 25-wavelength distance from the radiator. It has been established that, if the electrically conducting fluid is deposited on the opposite side of the plate, the shape of the (AFC)n, central frequency fn, and the phase characteristics of certain modes undergo changes that depend on whether the areas under the transducers are also covered with the fluid and on the distance between the transducers. No noticeable reflection of the modes from the plate edges has been recorded.

Properties of acoustic plate modes in YZ LiNbO3

Properties of acoustic plate modes (APMs) in YZ LiNbO₃ were calculated and measured. It was found that these modes have a dominant longitudinal component of mechanical motion concentrated near both surfaces of a plate. A delay line with double-electrode interdigital transducers (IDTs) was used for the measurements. Because of good matching to the load, strong triple-transit signals (TTS) were generated and insertion loss of about 7 dB was achieved at a frequency of about 131.4 MHz. Using water and glycerin solutions, insertion loss changes against viscosity were measured for this mode. In a viscosity range from about 1 mPa·s to 1000 mPa·s, an insertion loss change of about 16 dB was obtained. High sensitivity of viscosity measurement over a wide range makes the APMs in YZ LiNbO₃ attractive for application in viscosity sensors.

Scattering matrix approach to SHAPM delay line with long interdigital transducers on BT-cut quartz

Transfer function of a shear horizontal acoustic plate mode (SHAPM) delay line with long interdigital transducers (IDTs) was derived using the scattering matrix method. Analytical expressions for the admittance, scattering coefficients, and reflection coefficient of the periodic electrodes of the IDT, derived previously for SAWs, were used for numerical calculations. By matching the measured and calculated amplitude responses, the velocity, electromechanical coupling coefficient, and reflection coefficient of an aluminum electrode on BT-cut quartz were determined. It was shown that for a sufficiently thick aluminum layer, a reasonable insertion loss could be achieved in spite of a very small electromechanical coupling coefficient.

New acoustic plate modes with Quasi-Linear polarizations

New acoustic plate modes (APMs) with quasishear- vertical (QSV) and quasi-shear-horizontal (QSH) polarizations are found numerically in piezoelectric, nonpiezoelectric, and isotropic plates with free faces. Experimental verification of the modes is accomplished for ST,X-quartz plate, as an example. Similar to the modes with quasi-longitudinal displacement reported recently, the new counterparts exist when their velocities v(APM) are close to the velocities v(BAW) of the relevant bulk waves, the value of the dispersion slope dv(APM)/d(h/λ) is low enough (dv(APM)/d(h/λ) ~100 m/s, where h is thickness and λ is wavelength), and allowable plate thickness h/λ form a regular succession. All modes with quasi-linear polarization originate from generalized Lamb modes transformed at the aforementioned conditions.

Propagation of the Anisimkin Jr.' and quasi-longitudinal acoustic plate modes in low-symmetry crystals of arbitrary orientation

The Anisimkin Jr. (AN) acoustic plate mode having dominant and depth-independent longitudinal displacement (u(1) >> u(2), u(3); u(1) ≈ constant) is numerically found in tetragonal 4mm Li(2)B(4)O(7) crystal with one of the low-symmetry orientations (Euler angles 89°, 37°, 104°), as an example. The quasi-longitudinal (QL) modes with dominant and depth-dependent longitudinal displacement (u(1) >> u(2), u(3); u(1) ≠ constant) are experimentally detected along several propagation directions Θ in 128°y-LiNbO(3) plate, where Θ = 0°, 30°, 60°, and 90° with respect to the x-axis. Compared with more symmetrical plate materials and orientations, the displacement profiles of the AN and QL modes in lower-symmetry counterparts are qualitatively the same, but their phase profiles are more complicated. Moreover, like any acoustic wave, all plate modes in anisotropic crystals suffer from beam steering, in general. The power flow angles of the modes propagating in a fixed direction are different and depend on the mode order n.

Properties of shear horizontal acoustic plate modes in BT-Cut quartz

Properties of shear horizontal acoustic plate modes (SHAPMs) in BT-cut quartz were calculated and measured. A delay line with a long interdigital transducer, deposited on -50.5°YX90°-oriented quartz plate, was used for the measurements. For one of the SHAPMs, at a frequency of about 100.4 MHz, insertion loss, turnover temperature, and quadratic temperature coefficient of frequency of about 10 dB, 15°C, and -30 ppb/(°C)(2) in air, respectively, were obtained. Using water and glycerin solutions, insertion loss changes against dynamic viscosity were measured for this mode. In a viscosity range from about 1 mPa·s to 1000 mPa·s, an insertion loss change of about 14 dB was obtained.

Acoustic plate mode propagation and interaction with ultraviolet light in periodic AIN-on-sapphire structure

AlN overlay featuring periodic columnar structure fabricated by epitaxial lateral overgrowth technique leads to excitation of acoustic plate modes (APMs) not observed in overlays without such periodic structure. The measured velocities of acoustic plate modes propagating in AlN-on-sapphire structure were verified by numerical simulation. The APM velocity is strongly modulated by UV illumination at wavelengths from 240 to 365 nm, and the corresponding phase response is sensitive to both the UV power and the wavelength with maximum sensitivity of 3.0 ppm/(μW/cm2) at 240 nm.

Applicability of acoustic plate modes with quasi-longitudinal polarization to liquid parameterization

A class of acoustic waves suitable for liquid parameterization is demonstrated not to be restricted to acoustic modes with shear-horizontal polarization. Its substantial extension can be achieved by adding widespread plate modes with quasi-longitudinal (QL) polarization. Their investigation is performed by an example of 128°-rotated Y-cut LiNbO3 plates. The sensitivity of plate modes to the conductivity of liquid adjacent to one of the plate surfaces has been found to depend on a mode number, a plate thickness, and electric conditions on the opposite surface. It is demonstrated that their sensitivity to temperature is affected by the liquid viscosity. One of the QL modes existing in the 128°Y, X + 90° -LiNbO3 plate has been determined numerically. Its applicability for monitoring the electric characteristics of low-viscous liquid media has been examined experimentally.

General properties of the Anisimkin jr. plate modes

Acoustic plate modes of different orders n, having equal velocities v(n) close to that of the longitudinal BAW v(L), are numerically studied in crystals of different symmetries. Three families of the modes with v(n) ≈ v(L), each at relevant plate thickness h/λ = (h/λ)n, are found (h is the thickness, λ is the wavelength): the generalized Lamb mode with comparable longitudinal u1, shear-horizontal u2, and shear-vertical u3 displacements, the Anisimkin Jr. (AN) mode with u1 >> u2 and u3, and u1 ≈ constant ≠ 0 at any depth, and the quasilongitudinal (QL) mode with u1 > u2, and u3, but u1 ≠ constant over the plate thickness. Existence of the families does not depend on anisotropy or piezoelectric properties of the plate, but on the closeness of the mode velocity v(n) to the BAW velocity v(L), the value of the dispersion slope dv(n)/d(h/λ) at v(n) = v(L) and h/λ = (h/λ)n, and the proximity of the plate thickness (h/λ)n supporting the mode, to the thickness (h/λ)R providing transverse BAW resonance between plate faces. The Lamb modes approach v(n) = v(L) at irregular (h/λ)n far from resonance (h/λ)R and at large dv(n)/d(h/λ) ~10(3) m/s. The two other modes are characterized by lower dispersion dv(n)/d(h/λ) ≤ 10(3) m/s and regular (h/λ)n close to the resonance (h/λ)R. Because both modes have small vertical displacement on plate faces and propagate almost entirely within the crystals, they are attractive for liquid sensing.

Surface Generated Acoustic Wave Biosensors for the Detection of Pathogens: A Review

This review presents a deep insight into the Surface Generated Acoustic Wave (SGAW) technology for biosensing applications, based on more than 40 years of technological and scientific developments. In the last 20 years, SGAWs have been attracting the attention of the biochemical scientific community, due to the fact that some of these devices - Shear Horizontal Surface Acoustic Wave (SH-SAW), Surface Transverse Wave (STW), Love Wave (LW), Flexural Plate Wave (FPW), Shear Horizontal Acoustic Plate Mode (SH-APM) and Layered Guided Acoustic Plate Mode (LG-APM) - have demonstrated a high sensitivity in the detection of biorelevant molecules in liquid media. In addition, complementary efforts to improve the sensing films have been done during these years. All these developments have been made with the aim of achieving, in a future, a highly sensitive, low cost, small size, multi-channel, portable, reliable and commercially established SGAW biosensor. A setup with these features could significantly contribute to future developments in the health, food and environmental industries. The second purpose of this work is to describe the state-of-the-art of SGAW biosensors for the detection of pathogens, being this topic an issue of extremely importance for the human health. Finally, the review discuses the commercial availability, trends and future challenges of the SGAW biosensors for such applications.

Surface Generated Acoustic Wave Biosensors for the Detection of Pathogens: A Review

This review presents a deep insight into the Surface Generated Acoustic Wave (SGAW) technology for biosensing applications, based on more than 40 years of technological and scientific developments. In the last 20 years, SGAWs have been attracting the attention of the biochemical scientific community, due to the fact that some of these devices - Shear Horizontal Surface Acoustic Wave (SH-SAW), Surface Transverse Wave (STW), Love Wave (LW), Flexural Plate Wave (FPW), Shear Horizontal Acoustic Plate Mode (SH-APM) and Layered Guided Acoustic Plate Mode (LG-APM) - have demonstrated a high sensitivity in the detection of biorelevant molecules in liquid media. In addition, complementary efforts to improve the sensing films have been done during these years. All these developments have been made with the aim of achieving, in a future, a highly sensitive, low cost, small size, multi-channel, portable, reliable and commercially established SGAW biosensor. A setup with these features could significantly contribute to future developments in the health, food and environmental industries. The second purpose of this work is to describe the state-of-the-art of SGAW biosensors for the detection of pathogens, being this topic an issue of extremely importance for the human health. Finally, the review discuses the commercial availability, trends and future challenges of the SGAW biosensors for such applications.

Relaxation Effect of Acoustic Plate Modes Bordered with High Viscous Fluids

This study theoretically and experimentally investigated the relaxation of acoustic plate modes (APMs) under high viscous fluid loading. Partial wave theory was adopted to analyze acoustic waves that propagate in a piezoelectric plate and fluid, which was modeled as a Maxwellian fluid. A delay-line APM sensor consisted of input and output interdigital transducers (IDTs) with center frequency of 51 MHz was proven experimentally. By varying the ambient temperature, the insertion-loss saturation of APM, which was in contact with glycerol, was clear because of relaxation. Comparing the theoretical results with the experimental data yielded a high-frequency shear rigidity modulus of 2.5 ×108 N/m2, which was related to the relaxation effect of the fluid. The dependence of APM frequency on temperature was also obtained. The high linearity of the relationship revealed that the APM sensor could be used to detect the temperature of the surrounding fluid.

Properties of the Anisimkin Jr.' Modes in Quartz Plates

Dispersion curves, surface displacements, and displacement profiles over the plate thickness are numerically calculated for acoustic plate modes, propagating in fused and Y-rotated, X-cut quartz samples with thickness h/lambda in the range 0-4.5 (h, thickness; lambda, wavelength). The Anisimkin Jr.' modes with velocity v(n) close to that of longitudinal bulk wave v(L) and the dominant longitudinal displacement u1 distributed uniformly through the plate thickness are found in quartz crystals with cut angles mu = 120 degrees-140 degrees and plate thickness h/lambda = 0-0.36, 0.94-1.78, 2.32-3.08, and 3.64-4.44. The same modes in fused quartz are not found, except in a narrow region near zero thickness.